Interesting explanations for the abilities gained from the key items.
After 5 I saw the Devil Bringer's ability to absorb stuff as a weaker version of Rebellion's fusion ability, or just a trait of the Spardas like how Dante could absorb demon souls to gain styles. The angle of constantly trying to complete itself is neat but it kind of limits it so that once he'd unlocked his true DT he'd no longer be able to absorb stuff, which depending on how you interpret him still replacing his arm with breakers might not be true as of five.
I saw the Evil Legacy as just acting as a source of power that just strengthened Nero's existing astral projection abilities. (Remember he was able to manifest the spectral arm already when fighting Dante at the start, when he threw the giant stone sword at him).
Didn't see the Sephirophic fruit that way. I always thought it just gave him limited control over the plants Echidna spawned which he could only really manifest via destroying them, but that's a fairly reasonable explanation. Alternatively the game file says it holds the power that binds together demonic plants, so I saw it as absorbing that binding energy from the plants into itself.
As for the Aegis Shield, I saw it as not originally coming from Credo but something that was infused into him during his ascension ceremony that then influenced his form. Thus why Nero could absorb it (and the item file looks different from Credo's shield) without destroying Credo's own shield.
The Yamato one I agree with fully however.
Edit: For the Rusulka corpse, I saw it as him gaining an ability I'll call detection. Which merely resonates with nearby demonic power. A s a parallel to feelers helping search out sources of food.
